Presently, television receivers are being manufactured which incorporate therein means for displaying pictorial content in addition to that which is received from a broadcast studio or through a closed circuit or CATV cable system or from a video playback unit. These auxiliary presentations include items such as channel number and time of day. Since the use of these auxiliary presentations is relatively new in the art, most television receivers do not have equipment built in to provide such presentations. Thus, there are more than one hundred million television receivers in the United States alone which cannot display these auxiliary presentations such as time of day, channel number and the like.
There is disclosed in U.S. Pat. No. 3,728,480 which issued on Apr. 17, 1973 apparatus for displaying additional information in conjunction with information originating from a cooperative station such as a broadcast station, closed circuit television system, or a CATV network. In this patent, horizontal and vertical synchronization signals are extracted from the television receiver by employing a device in front of the receiver and attached thereto by, for example, a suction cup. This device includes a pick-up coil which extracts the magnetic field from the horizontal detection circuits which operate in step with the received synchronization signals and a photocell which is used to pick up a sixty Hertz signal component provided by a white rectangle or stripe at the bottom of the cathode ray tube of the receiver which would be broadcast by the cooperative station. This method is not entirely satisfactory in that the inductive coupling of the horizontal signal from a coil located on the cathode ray tube face is relatively weak and the use of a photocell to generate a vertical synchronization signal necessitates that the cooperative station generate a white marker at the vertical synchronization rate. Furthermore, it requires that a portion of the television screen be obstructed.
U.S. Pat. No. 3,728,480 also discloses what is referred to as a crowbar modulation circuit for overlaying auxiliary information on top of the information generated by the cooperative station. However, practical application of the techniques were never commercially employed because of the difficulty of extracting the synchronization signals from the received programs.